How to establish the best texture resolution?
There is no one rule. In my opinion it all depends on how big those textures will be presented on the screen in the final product.

3D rendering can be very demanding in terms of computing power. Large textures will use more resources than smaller ones. In order to succesfully create and manipulate scenes, all tricks that can yield acceptable outputs using less resources should be used. In other words: optimize the scene as much as you can, to fit within a realistic use of resources to get a satisfying outcome.

For starters, think that 4k image use 4 times more resources than a 2k one. If you can't perceive the benefit, then you probably don't need the extra resolution. 

If you have an object with a 4k texture that will only use a couple hundred pixels on the screen in the final product, you are indeed wasting resources.

Use large resolutions only for things that are going to be presented up close, or that will use a large section of the screen in the final product.